The invention is directed to gas generant compositions. In particular, the invention is directed to low solids gas generant compositions containing basic copper nitrate, and having a relatively low flame temperature.
Single use, i.e., xe2x80x9cone shotxe2x80x9d gas generators are well known in the art, and have become commonplace for many applications. In general, such gas generators are used to perform work in an emergency or in an a situation requiring the one-time production of a working gas. For such applications, the gas must be provided on demand in a consistent manner with high reliability. That is, the gas must be provided in the amount and at the pressure required, and the gas generator must operate with high reliability when generation of the gas is required. Typical applications include, but are not limited to, inflating automotive air bags, dispersing munitions from a cruise missile with air bags, inflating safety devices, such as buoyancy devices. e.g., life rafts and life preservers, inflating temporary structures, such as airplane escape slides, moving mechanical devices, such as pistons and rotary actuators, and providing inert gas for fire suppression, etc.
Although pressurized gas stored in a pressure vessel may be utilized in certain applications, pressurized gas sources are large and heavy. As a result, many applications may be performed more efficiently and more reliably using a pyrotechnic gas generating device that produces warm or hot gases from the combustion of a pyrotechnic gas generating material. In general, pyrotechnic gas generators produce more energy per unit mass and per unit volume than do compressed gas devices. They are also typically more reliable, as gas may leak out of the pressurized gas systems during storage, resulting in the release of an insufficient amount of gas when the device is finally operated.
The performance requirements of pyrotechnic gas generators vary in accordance with different applications, where the gas produced must meet certain requirements for temperature, toxicity, and corrosiveness. As the choice of pure pyrotechnic gas generators is limited by the selection of gas generant compositions, the development of a gas generant to meet certain performance criteria, such as burn rate, operating pressure, mechanical integrity of the gas generant grains, operational temperature range, and water content, is somewhat of an art. The development of a gas generant generally requires a number of compromises to meet those performance requirements in addition to requirements for gas temperature, toxicity, and corrosiveness. For example, the toxicity and corrosiveness of the effluent gases are of particular concern in many inflatable devices, such as where an inflatable device is used in a confined environment in which humans are present; e.g., automotive air bags. The pyrotechnic gas generants used in such devices often must compromise performance to provide an acceptably low toxicity and corrosiveness in the gas composition.
Inflatable devices also typically require a relatively cool gas to prevent damage to the material from which the inflatable device is fabricated. A relatively cool gas may also be required to keep the inflatable device fully inflated for sustained periods of time, depending on the temperature of the environment. Where the gas used to inflate an inflatable device is significantly hotter than the surrounding environment when the device is initially inflated, the pressure within the device will decrease shortly after the inflation is complete as the gas cools, resulting in at least a partial deflation of the inflated device. Extra gas may be added to the inflatable device to maintain the required inflation pressure after the gas within the device cools. However, the device may be over inflated when the hot gases are initially discharged into the device, potentially rupturing the inflatable device during inflation.
Some early prior art air bag inflators used a sodium azide/metal oxide based gas generant compositions to inflate the air bags. The sodium azide/metal oxide compositions burn at relatively cool temperatures, on the order of from about 1000xc2x0 to about 1200xc2x0 C., and have burn rates sufficiently fast to provide the required air bag inflation times with reasonable gas generant grain sizes and inflator operating pressures. However, those compositions also produce a large amount of unwanted solid combustion products, which, typically, account for about 60 percent of the initial weight of the composition, and include a large percentage of sodium oxide, a highly caustic and corrosive material capable of damaging lung tissues if inhaled in any significant quantity. As a result, filtration is required to remove the solid combustion products from the inflation gases. To provide a sufficiently cool gas, prior art pyrotechnic gas generators thus generally require complex filtration and heat sinking assemblies within the gas generator to remove unwanted solid combustion byproducts and heat from the gas before the gas exits the gas generator. The requirement to thoroughly filter out this toxic solid combustion product significantly adds to the cost and complexity of the filtration system within the sodium azide based air bag inflators. Moreover, sodium azide is highly toxic and hazardous to the environment, making the manufacture and the disposal of old or fired sodium azide based inflators costly and hazardous.
More recent prior art pyrotechnic air bag inflators use pyrotechnic gas generant compositions that are more environmentally friendly than sodium azide based compositions. However, to achieve the same performance as the original sodium azide based inflators in prior art xe2x80x9cnon-azidexe2x80x9d based gas generants generally requires a higher flame temperature than that of the original sodium azide based compositions, requiring additional heat sinking. Cooler burning non-azide based formulations are available, but typically have lower burning rates than azide formulations, and produce high levels of unwanted solid products of combustion, such that complex filtration is required. The prior art non-azide based gas generant formulations also tend to produce higher levels of toxic compounds in their effluent gases, such as, e.g., carbon monoxide, oxides of nitrogen, and hydrogen cyanide.
Hybrid inflators have been developed to mitigate the limitations of the newer non-azide based formulations. Hybrid inflators use a pressurized gas that is heated by a pyrotechnic gas generant to inflate the air-bag. The pressurized gas reduces the amount of gas generant required for the application, and provides additional cool gas to mix with the hotter gases provided by the gas generant composition, thus resulting in an overall lowering of the gas temperature. The pyrotechnic gas generant composition provides energy to the gas, allowing the inflator to meet weight and size requirements that cannot be met by compressed gas sources alone. Hybrid gas generators meet the gas temperature and particulate requirements of the air bag inflators at a lower cost than the first generation sodium azide based gas generators. However, hybrid inflators are more complex, and may be less reliable due to the use of pressurized gas. A purely pyrotechnic gas generator using a pyrotechnic composition that meets the performance, gas temperature, and toxicity requirements would be less expensive, less complex, and more reliable. However, a gas generant meeting all the air bag inflator requirements does not exist in the prior art.
A number of prior art second generation air bag gas generators use 5-amino tetrazole as a primary fuel in non-azide based gas generants. Most second generation gas generants provide adequate burn rates and operating pressures, but have flame temperatures as high as from about 2500xc2x0 to about 3000xc2x0 C. without the use of a coolant. For example, U.S. Pat. No. 5,035,757 to Poole discloses strontium nitrate as the primary oxidizer for a 5-amino tetrazole fuel in second generation gas generants. This composition is typically stoichiometric in oxidizer/fuel balance to minimize the formation of carbon monoxide and oxides of nitrogen, such as NO, NO2, and N2O, and has a number of desirable characteristics. In particular, with reasonably sized gas generant grains, it ignites easily and burns fast enough to allow for low operation pressures, i.e., from about 1000 to about 3000 psi. It also produces a relatively low volume of solids upon combustion, i.e., about 0.08 cubic centimeters of theoretical solid volume per gram of gas generant combusted, and uses relatively inexpensive ingredients.
However, the adiabatic flame temperature of a stoichiometric mixture of strontium nitrate and 5-amino tetrazole is about 2700xc2x0 C., when calculated using the thermochemical equilibrium combustion code PEPCODE, a commercially available computer program that calculates flame temperatures based upon the components of a composition. Such a flame temperature is about 1600xc2x0 C. higher than typical first generation sodium azide formulations. As a result, a substantial increase in the amount of heat sinking or the addition of a coolant is required to bring the gas temperature into a manageable range. Heat sinking increases the volume, weight, and cost of the gas generator, and tends to increase the gas toxicity above acceptable levels. The increase in toxicity is due, at least in part, to catalytic interactions between the hot heat sink surface and the gases, forming toxic species, such as oxides of nitrogen and hydrogen cyanide. In addition, the amount of heat sinking required to provide a decrease in temperature of over 1000xc2x0 C. in the gas results in a significant increase in the temperature of the device after firing because of the energy absorbed by the heat sink. As a result, there is a significant burn risk to the occupants of a vehicle equipped with such an inflator.
Prior art coolants can add significant quantities of solids to gas generant combustion effluents, requiring additional filtration and, thus, additional unit cost and complexity. Coolants also tend to significantly reduce the burn rate of the gas generant composition when used in quantities sufficient to provide useful cooling of the flame, increasing operating pressure requirements, and decreasing gas generant grain sizes both of which add to unit cost. Some examples of typical coolants used are metal carbonates and bicarbonates, such as, e.g., sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, magnesium carbonate, and calcium carbonate, and metal oxides, such as, e.g., aluminum oxide, magnesium oxide, zinc oxide, and iron oxide.
For example, U.S. Pat. No. 6,051,158 to Taylor et al. discloses a chemical coolant formulation for use with air bag inflators. The formulation includes a xe2x80x9cfirst coolant ingredientxe2x80x9d that endothermically decomposes when contacted by a hot gas to form a cooling gas and a solid slag component. Disclosed coolants include metal carbonates, metal hydroxide, and hydrated metal salts. Disclosed hydrated metal salts are limited to MgSO4.7H2O and MgCl2.6H2O. However, for use in an inflator, the slag component requires extensive filtering of the generated gas.
U.S. Pat. Nos. 5,735,118 and 6,039,820 to Hinshaw et al. disclose the use of alkaline earth metal and transition metal complexes as gas generating compositions. The disclosed compositions comprise a metal cation and a neutral ligand containing hydrogen and nitrogen with one or more oxidizing anions to balance the charge of the complex. The preferred neutral ligands are ammonia, substituted ammonia ligands, such as hydrazine, and substituted hydrazine ligands. Optionally, burn rate enhancers, slag formers, and coolants such as magnesium hydroxide, cupric oxalate, boric acid, aluminum hydroxide, and silicotungstic acid, may be used. As the complexes produce a significant amount of slag during combustion, gas filtration is required.
U.S. Pat. No. 3,806,461 to Hendrickson et al. discloses gas generating compositions comprising a mixture of potassium perchlorate, cupric oxalate and a relatively small amount of a polymeric fuel binder to provide a relatively cool gas. The patent discloses that, when heated, cupric oxalate decomposes exothermically to produce copper, cuprous oxide, carbon dioxide, and carbon monoxide, providing the driving force that results in a relatively high burning rate. The potassium perchlorate is used to burn the polymeric fuel binder, and to remove carbon monoxide by oxidizing that toxic gas to carbon dixoide. However, although the decomposition of cupric oxalate produces a cool gas, the decomposition of cupric oxalate is actually endothermic, and cupric oxalate alone does not provide the energetic output typically available from pyrotechnic compositions.
U.S. Pat. Nos. 5,542,998 and 5,542,999 to Bucerius et al. disclose gas generating mixtures for rescue and air bag systems, as well as rocket and tubular weapon drive systems. The mixtures contain high nitrogen, low carbon fuels of GZT, TAGN, NG, and NTO, a catalyst for reducing pollutant gases, and an oxidizer of basic copper nitrate, Cu(NO3)2.3Cu(OH)2, and optionally, Fe2O3 as a coolant to provide a cold, rapid combustion and a high gas output.
U.S. Pat. No. 5,608,183 to Barnes et al. discloses gas generant compositions containing a nitrate salt of a polyamine or an alkyl-diamine as a fuel and basic copper nitrate and/or cobalt triamine nitrate as an oxidizer. Disclosed fuels include nitrate salts of urea, guanidine, aminoguanidine, diaminoguanidine, semicarbizide, ethylene diamine, 1,3-propane diamine, and 1,2-propane diamine. The compositions produce at least 2 moles of gas per 100 grams of generant, and burn at a temperature of no more than about 2000xc2x0 C.
European Patent Publication No. EP 0 949 225 to Roedig et al. discloses azide-free gas generating compositions containing a fuel mixture and an oxidant mixture. The fuel mixture contains a guanidine compound, a heterocyclic organic acid, and, optionally, an additional fuel. The oxidizer mixture contains at least one transition metal oxide, basic copper nitrate, and a metal perchlorate, ammonium perchlorate, an alkali metal nitrate, and/or an alkaline earth metal nitrate.
International Patent Publication No. WO 99/31029 of Fonblanc et al. discloses pyrotechnic gas generating mixtures consisting essentially of an epoxy or silicone resin based cross-linkable reducing binder, an oxidizer of ammonium perchlorate and a sodium nitrate chlorine scavenger, and energetic additives consisting of a cupric compound, i.e., cupric oxide and basic copper nitrate, and a nitrogenated organic compound, i.e., nitroguanidine and guanidine nitrate. The disclosed mixtures are said to burn at moderate temperatures, generating nitrogen rich gases that are xe2x80x9cpoorxe2x80x9d in nitrogen oxides and carbon monoxide.
U.S. Pat. No. 5,882,036 to Moore et al. discloses a hybrid inflator using a gas generant of basic copper nitrate, hexamine cobalt nitrate, guanidine nitrate, and guar gum.
U.S. Pat. No. 5,989,367 to Zeuner et al. discloses azide-free gas generating mixtures of an oxidizer of ammonium nitrate and ammonium perchlorate, an energy rich fuel of guanidine nitrate, nitroguanidine, triaminoguanidine, urea nitrate, nitrourea, pentaerythritol, tetranitrate, nitrotriazalone, hexogen, octogen, and mixtures thereof, and a combustion modifier of a transition metal oxide, hydroxide, nitrate, carbonate, or organo-metallic compound. Disclosed combustion modifiers include basic copper nitrate.
U.S. Pat. No. 6,077,372 to Mendenhall et al. discloses ignition enhanced gas generant materials in which an ignition composition is combined with a solvent and applied to a gas generant material. Disclosed ignitor composition fuels include aluminum, boron, and magnesium. Disclosed ignitor material oxidizers include alkali metal and alkaline earth metal nitrates, chlorates, and perchlorates, ammonium nitrate, ammonium perchlorate, and basic copper nitrate. The gas generant materials to which the ignition composition is applied include those containing nitrogen-containing organic compounds or tetrazole complexes as fuels, such as guanidine nitrate, aminoguanidine nitrate, triaminoguanidine nitrate, nitroguanidine, dicyandiamide, triazalone, nitrotriazalone, tetrazoles, and tetrazole complexes of copper, cobalt, and zinc. Oxidizers for the disclosed gas generant materials include ammonium nitrate and basic copper nitrate. The patent also discloses the use of a metal, as an additional fuel, and a metal oxide as a burn rate enhancer and slag producer.
Therefore, a need exists for an energetic, pyrotechnic gas generating composition that generates cool, non-toxic gases on combustion with the production of low levels of solids. The present invention provides such a composition.
The invention is directed to a low-solids gas generating composition having a relatively low flame temperature and to a method of generating a gas with low solids. The compositions of the invention comprise a mixture of a basic copper nitrate oxidizer and a fuel selected for the group consisting of 5-nitro-uracil, guanidine 5-nitro-uracil salt, ammonium 5-nitro-uracil salt, aminoguanidine 5-nitro-uracil salt, hydrazine 5-nitro-uracil salt, triamino 5-nitro-uracil salt, guanidine 5-nitro-barbituric acid salt, ammonium 5-nitro-barbituric acid salt, hydrazine 5-nitro-barbituric acid salt, aminoguanidine 5-nitro-barbituric acid salt, triaminoguanidine 5-nitro-barbituric acid salt, and mixtures thereof, wherein the oxidizer-fuel mixture is within about 4 percent of stoichiometric balance, and produces no more than about 0.06 cubic centimeters of solids per gram of gas generating composition on combustion. The compositions of the invention may further comprise at least one of cupric oxalate hemi-hydrate as a coolant, sub-micron fumed silica, and graphite. Preferably, the gas generant is in the form of pressed pellets, grains, or granules.
Preferred gas generating compositions in accordance with the invention include the following:
Mixtures comprising 5-nitro-uracil, basic copper nitrate, and, optionally, fumed silica, where the mixture preferably comprises from about 17.2 to about 39.3 percent 5-nitro-uracil, from about 82 to about 60.1 percent basic copper nitrate, and up to about 0.8 percent, preferably from about 0.8 to about 0.6 percent, fumed silica;
Mixtures comprising 5-nitro-uracil, basic copper nitrate, and, optionally, fumed silica and/or anhydrous activated Al2O3, where the mixture preferably comprises from about 16.5 to about 37.7 percent 5-nitro-uracil, from about 78.7 to about 57.7 percent basic copper nitrate, up to about 0.8 percent, preferably from about 0.8 to about 0.6 percent, fumed silica, and up to about 4 percent anhydrous activated Al2O3;
Mixtures comprising guanidine 5-nitro-uracil salt, basic copper nitrate, and, optionally, fumed silica, where the mixture preferably comprises from about 13.6 to about 32.3 percent guanidine 5-nitro-uracil salt, from about 84.7 to about 66.4 percent basic copper nitrate, and up to about 1.7 percent, preferably from about 1.7 to about 1.3 percent, fumed silica;
Mixtures comprising ammonium 5-nitro-uracil salt, basic copper nitrate, and, optionally, fumed silica, where the mixture preferably comprises from about 15.3 to about 35.6 percent ammonium 5-nitro-uracil salt, from about 83 to about 63 percent basic copper nitrate, and up to about 1.7 percent, preferably from about 1.7 to about 1.3 percent, fumed silica;
Mixtures comprising aminoguanidine 5-nitro-uracil salt, basic copper nitrate, and, optionally, fumed silica, where the mixture preferably comprises from about 22.8 to about 32.7 percent aminoguanidine 5-nitro-uracil salt, from about 75.7 to about 66 percent basic copper nitrate, and up to about 1.5 percent, preferably from about 1.5 to about 1.3 percent, fumed silica;
Mixtures comprising hydrazine 5-nitro-uracil salt, basic copper nitrate, and, optionally, fumed silica, where the mixture preferably comprises from about 15.5 to about 36 percent hydrazine 5-nitro-uracil salt, from about 82.8 to about 62.7 percent basic copper nitrate, and up to about 1.7 percent, preferably from about 1.7 to about 1.3 percent, fumed silica;
Mixtures comprising triaminoguanidine 5-nitro-uracil salt, basic copper nitrate, and, optionally, fumed silica, where the mixture preferably comprises from about 14.2 to about 33.4 percent triaminoguanidine 5-nitro-uracil salt, from about 84.4 to about 65.3 percent basic copper nitrate, and up to about 1.7 percent, preferably from about 1.7 to about 1.3 percent, fumed silica;
Mixtures comprising guanidine 5-nitro-barbituric acid salt, basic copper nitrate, and, optionally, fumed silica, where the mixture preferably comprises from about 15.7 to about 36.5 percent guanidine 5-nitro-barbituric acid salt, from about 82.6 to about 62.3 percent basic copper nitrate, and up to about 1.7 percent, preferably from about 1.7 to about 1.2 percent, fumed silica;
Mixtures comprising ammonium 5-nitro-barbituric acid salt, basic copper nitrate, and, optionally, fumed silica, where the mixture preferably comprises from about 18.4 to about 41.8 percent ammonium 5-nitro-barbituric acid salt, from about 80 to about 57 percent basic copper nitrate, and up to about 1.6 percent, preferably from about 1.6 to about 1.2 percent, fumed silica;
Mixtures comprising hydrazine 5-nitro-barbituric acid salt, basic copper nitrate, and, optionally, fumed silica, where the mixture preferably comprises from about 18.4 to about 41.7 percent hydrazine nitro-barbituric acid salt, from about 80 to about 57.2 percent basic copper nitrate, and up to about 1.6 percent, preferably from about 1.6 to about 1.1 percent, fumed silica;
Mixtures comprising aminoguanidine 5-nitro-barbituric acid salt, basic copper nitrate, and, optionally, fumed silica, where the mixture preferably comprises from about 15.8 to about 36.7 percent aminoguanidine 5-nitro-barbituric acid salt, from about 82.5 to about 62.1 percent basic copper nitrate, and up to about 1.7 percent, preferably from about 1.7 to about 1.2 percent, fumed silica;
Mixtures comprising triaminoguanidine 5-nitro-barbituric acid salt, basic copper nitrate, and, optionally, fumed silica, where the mixture preferably comprises from about 16 to about 37.1 percent triaminoguanidine 5-nitro-barbituric acid salt, from about 82.4 to about 61.7 percent basic copper nitrate, and up to about 1.6 percent, preferably from about 1.6 to about 1.2 percent, fumed silica;
Mixtures comprising basic copper nitrate, cupric oxalate hemi-hydrate, and, optionally, 5-nitro-uracil, where the mixture preferably comprises from about 50 to about 27.9 percent basic copper nitrate, about 50 percent cupric oxalate hemi-hydrate, and up to about 22.1 percent 5-nitro-uracil;
Mixtures comprising basic copper nitrate, cupric oxalate hemi-hydrate, and, optionally, guanidine 5-nitro-uracil salt, where the mixture preferably comprises from about 50 to about 31.7 percent basic copper nitrate, about 50 percent cupric oxalate hemi-hydrate, and up to about 18.3 percent guanidine 5-nitro-uracil salt; and
Mixtures comprising basic copper nitrate, guanidine nitrate, 5-nitro-uracil, cupric oxalate hemi-hydrate, graphite, and, optionally, fumed silica, where the mixture preferably comprises about 40.4 percent basic copper nitrate, about 6.6 percent guanidine nitrate, about 18.6 percent 5-nitro-uracil, about 33.3 percent cupric oxalate hemi-hydrate, about 0.3 percent graphite, and about 0.8 percent fumed silica.
The method of the invention comprises preparing a mixture comprising a basic copper nitrate oxidizer, a fuel selected from the group consisting of 5-nitro-uracil, guanidine 5-nitro-uracil salt, ammonium 5-nitro-uracil salt, aminoguanidine 5-nitro-uracil salt, hydrazine 5-nitro-uracil salt, triamino 5-nitro-uracil salt, guanidine 5-nitro-barbituric acid salt, ammonium 5-nitro-barbituric acid salt, hydrazine 5-nitro-barbituric acid salt, aminoguanidine 5-nitro-barbituric acid salt, triaminoguanidine 5-nitro-barbituric acid salt, and mixtures thereof, and, optionally, at least one of cupric oxalate hemi-hydrate as a coolant, sub-micron fumed silica, and graphite, wherein the oxidizer-fuel mixture is within about 4 percent of stoichiometric balance; combusting the mixture, thereby producing a gas and no more than about 0.06 cubic centimeters of solids per gram of gas generating composition on combustion.
Unless otherwise stated, all references to xe2x80x9cpercentxe2x80x9d or xe2x80x9c%xe2x80x9d refer to percent by weight based on the total weight of the composition.
As used herein, the term xe2x80x9cstoichiometric balancexe2x80x9d means that the ratio of oxidizer to fuel is such that upon combustion of the composition all of the fuel is fully oxidized, and no excess of oxygen is produced. A xe2x80x9cnear stoichiometric balancexe2x80x9d is one in which the ratio of oxygen mass surplus or deficit to total mixture mass is within about four percent of a stoichiometric balance.
As used herein, the terms xe2x80x9clow solidsxe2x80x9d and xe2x80x9clow levels of solidsxe2x80x9d mean that, upon combustion, the gas generant produces substantially lower solids than the 60 percent solids produced on combustion by gas generants used in prior art pyrotechnic inflators, such as sodium azide based inflators. The gas generants of the invention typically produce less than about 60 percent solids, and preferably less than about 0.06 cubic centimeters of solids per gram of gas generant. This is advantageous in that it minimizes or eliminates the need for a filter in the inflator, thus, simplifying inflator design.
All flame temperatures referred to herein are adiabatic flame temperatures calculated with the thermochemical equilibrium combustion code PEPCODE.
The present invention is directed to low solids, high gas producing gas generant compositions that produce essentially non-toxic gases upon combustion. The compositions of the invention comprise a primary fuel and primary oxidizer, and may further include additives, such as, e.g., burn rate catalysts, coolants, anti-oxidants, and manufacturing aides. The compositions of the invention produce a low volume of solids upon combustion, and, thus, when used in a gas generator, the gas generator requires little or no filtration.
The flame temperatures of the gas generant compositions of the invention are generally below about 2000xc2x0 C., and preferably, below about 1600xc2x0 C. Most non-azide gas generants that meet both the performance requirements for burn rate, operating pressure, toxicity, and low volume of unwanted solid combustion byproducts have adiabatic flame temperatures that are generally greater than about 2000xc2x0 C. With the present invention, flame temperatures below 1000xc2x0 C. may be obtained in gas generants that support continuous combustion at ambient temperature and pressure, which is a highly desirable feature in certain applications, such as, e.g., in the inflation of devices which require sustained pressurization, but do not require extremely fast inflation, such as life rafts, other such buoyancy devices, and emergency slides for air planes. A gas generating composition that can burn at ambient allows for very simple, light weight containers to be used for the gas generator case, providing gas generators having masses and volumes less than half that of an equivalent pressurized gas system.
The primary oxidizer in the preferred embodiments is basic copper nitrate, Cu4(OH)6(NO3)2, which is a unique oxidizer. Basic copper nitrate produces a very low volume of solids upon combustion, about 53 weight percent copper, based on the total original weight of basic copper nitrate when combusted in a gas generant with a fuel to oxidizer balance that is near stoichiometric. For example, basic copper nitrate, when thermally decomposed in an inert atmosphere to pure copper, will leave 53 percent by weight copper metal residue. The copper produced during combustion is very easy to trap within a gas generator due to its relatively high density of 8.9 grams per cubic centimeter. Generally, because of the flame temperature, the copper is in the liquid state in the gas generator due to its low melting point of 1083xc2x0 C., which further simplifies trapping, as the liquid tends to slag and plate out on cool surfaces inside the gas generator more easily than solids or gases. As a result, no separation or filtration above and beyond that provided by the interior walls of the combustion chamber is generally required because the copper xe2x80x9cslagsxe2x80x9d up in the combustion chamber forming a xe2x80x9cclinkerxe2x80x9d. Any remaining copper in the gas stream generally separates due to the forces acting on the copper as the gas turns and weaves through the combustion chamber making its way to the exit orifices of the device. The residual copper simply separates out because of its high density relative to the gases, and plates out on the inside walls of the gas generator. Compared to other oxidizers, basic copper nitrate forms relatively cool burning gas generant compositions, which is highly desirable in applications such as inflatable devices, where high temperature gases can damage the inflatable device material.
In the present invention, the primary fuel is selected to minimize the volume of solids produced upon combustion, and should be selected and balanced in proportions sufficient to provide a low level of gas toxicity. U.S. Pat. No. 5,780,768 to Knowlton, et al, the teachings of which are incorporated herein by reference to the extent necessary to supplement this specification, provides a good description of how to balance the fuel and oxidizer amounts in the composition to minimize gas toxicity. The constituents of the composition are selected to produce an output gas consisting essentially of carbon dioxide, nitrogen, water, and oxygen. Any halogens or sulfur contained in the composition should have an equimolar or greater amount of a corresponding alkali metal salt in the composition to remove any halogen or sulfur compounds produced during combustion by forming the corresponding alkali metal halogen or sulfur containing salt, which may then be separated as a solid. For example sodium nitrate or sodium carbonate in the original composition will be converted to sodium chloride or sodium sulfide or sulfate in a solid form, which is then separated from the generated gases. Again, this is described in detail in U.S. Pat. No. 5,780,768. The oxygen balance, i.e., the ratio of oxidizer to fuel in the composition should be within about four percent of stoichiometric balance as described in U.S. Pat. No. 5,780,768. Basic copper nitrate is preferably combusted to form copper, but may be combusted in the presence of excess oxidizer to form cuprous oxide and/or cupric oxide or a combination copper and cuprous oxide and/or cupric oxide. In the case of excess oxidizer, the limit of within about four percent of stoichiometric balance applies to that above and beyond what is required to combust all of the copper to cuprous or cupric oxide. That is, sufficient oxidizer must be added to burn all the carbon to CO2, all of the hydrogen to H2O, and all the copper to CuO before counting the excess oxidizer toward the four percent of stoichiometric balance limit.
For a mixture comprising, for example, 5-nitro-uracil and basic copper nitrate amounts of each component required to obtain an oxygen balance of plus or minus four percent of stoichiometric are determined as follows:
The amount of oxygen required to oxidize one gram of 5-nitro-uracil stoichiometrically, forming carbon dioxide, nitrogen and water, is 0.5605 grams, which corresponds to 5.5 moles of oxygen per mole of 5-nitro-uracil, C4N3H3O4.
Upon decomposition to copper, basic copper nitrate produces about 0.3 grams of oxygen per gram of basic copper nitrate, which corresponds to 9 moles of atomic oxygen per mole of basic copper nitrate, Cu4(OH)6(NO3)2. On decomposition and oxidation to copper oxide, basic copper nitrate produces about 0.167 grams of atomic oxygen per gram of basic copper nitrate, which corresponds to about 5 moles of oxygen per mole of basic copper oxide.
The lower limit for an oxygen balance of minus four percent with respect to stoichiometric is calculated using the value of 0.3 for the oxygen produced by basic copper nitrate decomposed to copper. In the following equations, 5NU represents 5-nitro-uracil, and BCN represents basic copper nitrate.
Mass fraction of BCNxc3x970.3xe2x88x92Mass fraction of 5NUxc3x970.5605=xe2x88x920.04,
where the mass fraction of BCN=1xe2x88x92mass fraction of 5NU. Therefore,
(1xe2x88x92Mass fraction of 5NU)xc3x970.3xe2x88x92mass fraction of 5NUxc3x970.5605=xe2x88x920.04
Solving those two equations yields composition mass percentages of about 39.5 percent 5-nitro-uracil and 60.5 percent basic copper nitrate for a composition having an oxygen balance of four weight percent less than the stoichiometric balance.
The upper limit of an oxygen balance of plus four percent with respect to stoichiometric is calculated using the value of 0.1667 grams of oxygen produced by the decomposition of one gram of basic copper nitrate to copper oxide.
Mass fraction of BCNxc3x970.1667xe2x88x92mass fraction of 5NUxc3x970.5605=+0.04,
Where the mass fraction of BCN=1xe2x88x92Mass fraction of 5NU. Therefore,
xe2x80x83(1xe2x88x92mass fraction 5NU)xc3x970.1667xe2x88x92mass fraction 5NUxc3x970.5605=+0.04.
Solving those two equations yields compositions having mass percentages of about 17.4 mass percent 5-nitro-uracil and 82.6 mass percent basic copper nitrate for a composition having an oxygen balance of four weight percent greater than the stoichiometric balance.
As an oxidizer, basic copper nitrate is stable when used in gas generant formulations with other compatible materials, but will react with acidic materials and with materials that are strong Lewis bases, such as compounds containing nitrogen with available free electron pairs, including, e.g., organic amines, hydrazides, azides, and imines. Acidic materials react with the copper hydroxide contained in the basic copper nitrate, and attack the basic copper hydroxide, forming the copper salt of the acid and water. Strong Lewis bases tend to act as ligands, attaching to the copper cation of the copper nitrate molecules in the basic copper nitrate crystal, which displaces copper hydroxide from the crystal structure. When displaced from the crystal structure, the copper hydroxide typically decomposes into cupric oxide and water. In each case, the water weakens the gas generant grains, causing the grains to lose structural integrity. Additionally, basic copper nitrate will decompose to copper oxide and nitric acid in the presence of a sufficiently large quantity of hot water. As a result, reaction with either a strong Lewis acid or a strong Lewis base will result in additional decomposition of the remaining basic copper nitrate in the presence of heat, making the list of available ingredients that are compatible with basic copper nitrate significantly shorter than the that for other oxidizers. Thus, the selection of fuels and additives for use in gas generant compositions containing basic copper nitrate as the oxidizer must be made carefully.
The following non-limiting examples are merely illustrative of the preferred embodiments of the present invention, and are not to be construed as limiting the invention, the scope of which is defined by the appended claims. All percentages are given in weight or mass percent.